Method and system for automatic gain control in a satellite communications system

ABSTRACT

An apparatus for providing automatic gain control for use in a satellite terminal of a satellite communication system capable of transmitting a plurality of different modes of data. The apparatus includes a demodulator circuit having an analog to digital converter; a first variable attenuator having an attenuation value set on the basis of a measured power level of a predetermined data signal; and a second variable attenuator having an attenuation value set on the basis of the mode of data being received by the satellite terminal, where each of the data modes have a corresponding predetermined attenuation value associated therewith which is utilized as the attenuation value of the second variable attenuator when the satellite terminal receives the data mode.

RELATED APPLICATIONS

[0001] This application claims priority under 35 U.S.C. §119(e) toprovisional application Ser. No. 60/260,840 filed Jan. 10, 2001.

FIELD OF THE INVENTION

[0002] The present invention relates to a method and system forimplementing an automatic gain control (AGC) technique for use insatellite communication systems and more particularly, to a costeffective AGC technique capable of compensating for wide variations inthe input signal power level.

BACKGROUND OF THE INVENTION

[0003] The use and need for AGC techniques in receivers utilized insatellite communication systems in order to compensate for variations inthe power level of the received signal is well known in the art.Generally speaking, the AGC portion of the receiver functions tomaintain the power level of the received/incoming signal within somepredetermined range.

[0004] Typically, satellite receivers utilizing current technologyemploy digital demodulators which function to downconvert and demodulatethe incoming data signal, and which utilize A/D converters to convertthe incoming analog data into a digital format. In operation, such A/Dconverters included in the demodulator require a fixed number of bits soas to provide the necessary resolution so as to allow the carrierrecovery loops contained in the demodulator to function properly (e.g.,minimize signal quantization errors). The number of bits the A/Drequires to provide the necessary resolution can be readily determinedbased on system requirements. The remaining bits of the A/D converterare utilized to compensate for variations in the power level of theincoming data signal (i.e., the dynamic range of the input data signal).Accordingly, if the input signal is expected to have a large dynamicrange, then additional bits of the A/D must be dedicated to handling thevariations in the dynamic range.

[0005] In prior art systems, when additional bits were necessary tocompensate for an increase in the dynamic range of the input signal, thesolution was simply to increase the number of bits of the A/D converter.As such, the A/D converter would have the requisite bits necessary toprovide the desired resolution, as well as the requisite bits necessaryto handle the dynamic range requirements.

[0006] However, as the data rates utilized by today's communicationsystems continue to increase, especially so with satellite communicationsystems, adding additional bits to the A/D converter to handle anincrease in dynamic range is no longer a feasible solution. For example,in a system allocating 5 bits of the A/D converter for signalquantization resolution, and which requires the ability to compensatefor a 30 db dynamic range variation with respect to the input signal, anadditional 5 bits are necessary. Thus, a 10 bit A/D converter would berequired. However, such high resolution A/D converters operating at highdata rates (e.g., 800 MHz) would be exceedingly expensive, and clearlycould not be utilized in any commercially viable product/system.

[0007] Accordingly, there exists a need for an AGC technique andimplementation that allows for the compensation of a large dynamic rangewith respect to the input signal without requiring an increase in theresolution capabilities of the A/D converter contained in thedemodulator of the communication system.

SUMMARY OF THE INVENTION

[0008] The present invention relates to a method and system forimplementing automatic gain control in a communication system thatallows for the compensation of a large dynamic range regarding the inputsignal without requiring an increase in the resolution capabilities ofthe A/D converter contained in the demodulator of the communicationsystem.

[0009] More specifically, the present invention relates to an apparatusfor providing automatic gain control for use in a satellite terminal ofa satellite communication system capable of transmitting a plurality ofdifferent modes of data. The apparatus includes a demodulator circuithaving an analog to digital converter; a first variable attenuatorhaving an attenuation value set on the basis of a measured power levelof a predetermined data signal; and a second variable attenuator havingan attenuation value set on the basis of the mode of data being receivedby the satellite terminal, where each of the data modes has acorresponding predetermined attenuation value associated therewith whichis utilized as the attenuation value of the second variable attenuatorwhen the satellite terminal receives the data mode.

[0010] In addition, the present invention relates to a method forproviding automatic gain control for use in a satellite terminal of asatellite communication system capable of transmitting a plurality ofdifferent modes of data. The method comprises the steps of: (1)measuring a power level of a predetermined data signal received by thesatellite terminal, (2) adjusting an attenuation value of a firstvariable attenuator on the basis of the measured power level of thepredetermined data signal, (3) adjusting an attenuation value of asecond variable attenuator on the basis of the mode of data beingreceived by the satellite terminal, where each of the data modes has acorresponding predetermined attenuation value associated therewith whichis utilized as the attenuation value of the second variable attenuatorwhen the satellite terminal receives the data mode. As a result of theforegoing method, the first variable attenuator and the second variableattenuator are operative for maintaining the input power level to ananalog to digital converter contained in a demodulator of the satelliteterminal within a predetermined range.

[0011] As described below, the system and method of providing automaticgain control in accordance with the present invention provides importantadvantages over prior art devices. Most importantly, the AGC system andmethod of the present invention allows for the compensation/processingof a wide dynamic range regarding the input signal without requiring theuse of a fast, high-order bit A/D converter in the demodulator. Asnoted, the use of such an A/D converter (assuming it is available) wouldrender the overall system prohibitively expensive due to the cost ofsuch an A/D converter. In contrast, as the present invention allows forthe compensation of a large dynamic range utilizing a low-cost, readilyavailable A/D converter, the present invention results in a commerciallyviable system.

[0012] Additional advantages of the present invention will becomeapparent to those skilled in the art from the following detaileddescription of exemplary embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a block diagram of an exemplary satellite terminal 10utilized in a satellite communication system.

[0014]FIG. 2 is an exemplary block diagram of the IF subsystem moduleillustrated in FIG. 1.

[0015]FIG. 3 is an exemplary block diagram of the VGA controllercontained in the modem subsystem, which is utilized to generate the VGAcontrol signal depicted in FIG. 2.

[0016]FIG. 4 is an exemplary block diagram of the PGA controllercontained in the modem subsystem, which is utilized to generate the PGAcontrol signal depicted in FIG. 2.

[0017] The invention itself, together with further objects and attendantadvantages, will best be understood by reference to the followingdetailed description, taken in conjunction with the accompanyingdrawings.

DETAILED DESCRIPTION OF THE DRAWINGS

[0018] The following detailed description of the novel AGC techniqueutilized in a satellite communication system sets forth exemplaryembodiments of the present invention. It is noted, however, that thepresent invention as claimed herein is not intended to be limited to thespecific embodiments disclosed in the following discussion. Clearlyother implementations of the novel AGC technique for use with satellitecommunication systems, as well as other communication systems, arepossible.

[0019]FIG. 1 is a block diagram of an exemplary satellite terminal 10utilized in a satellite communication system. As explained in detailbelow, the AGC technique of the present invention is implemented in thesatellite terminal 10. Referring to FIG. 1, the satellite terminalcomprises an antenna 12, an outdoor unit (ODU) 14 and an indoor unit(IDU) 16. In operation, the incoming data signal is coupled to the ODU14 via the antenna 12. The ODU 14 functions to perform preliminarydownconversion of the incoming signal, and then couples the signal tothe IDU 16 via an interfacility link 17. The IDU comprises an IFsubsystem 18 and a modem subsystem 20.

[0020] As explained in detail below, the IF subsystem 18 and the modemsubsystem 20 operate to implement the AGC technique of the presentinvention. Generally speaking, the IF subsystem 18 performs AGC,downconverts and digitizes the signal received from the ODU, so as togenerate a stream of data samples 22. The data samples output by the IFsubsystem 18 are coupled to the modem subsystem 20, which functions todemodulate the sampled data 22 and to generate AGC control signals(i.e., a VGA control signal 33 and a PGA control signal 23) which arecoupled to the IF subsystem 18.

[0021] The operation of the AGC technique of the present invention isnow described. As stated above, one objective of the present inventionis to allow for the use of the low cost A/D converter whilesimultaneously allowing for a wide dynamic range variation on the inputsignal. This is accomplished by the present invention by utilizing apriori knowledge regarding the power level of the incoming signals(based on the type/mode of signal) to continually adjust attenuators soas to maintain the power level range of the signal being fed to the A/Dconverter of the demodulator within a predetermined range. Bydynamically changing the attenuation levels in the foregoing manner, itis possible to accommodate/process input signals having significantlylarge dynamic range variations utilizing a low cost A/D converter.

[0022] It is noted that the operation of the present invention isdescribed in conjunction with the operation of a satellite communicationsystem. However, it is possible to utilize the novel AGC controltechnique with any s system having a priori knowledge of the power levelrange of the incoming data signals.

[0023] Prior to a detailed discussion of the invention, a brief overviewof the data signals generated by the satellite communications system ishelpful. The system employs essentially four distinct signaling modes,which are: (1) Beacon mode (BCN); (2) CONUS4 (C4) mode; CONUS3 (C3) modeand point-to-point (PTP) mode, and transmits these data signals to thesatellite terminal utilizing a Ka-band downlink (e.g., 19.9-20.2 GHz).The data signals are transmitted to the satellite terminals in a TDMAframe format, in which the data signals are transmitted at defined timeswith the frame. However, each satellite terminal in the system knows thesignaling mode composition of a given frame at least one frame inadvance of the receipt of the given frame. In other words, the satelliteterminal knows which signal modes are to be included in a given frameprior to the actual transmission of the frame of data.

[0024] It is further noted that the power levels associated with thefour signaling modes of the system are different. Specifically, thebeacon signal is not power controlled by the system. As such, the beaconsignal received at the satellite terminal is subject to power levelvariations. The C3, C4 and the PTP signals are power controlled, and aretransmitted having different power levels than the beacon signal. Forexample, the C3 and C4 signals may be transmitted at signal levels 6 dbhigher than the beacon signal, while the PTP signal may be transmittedat a level 12 db over the CONUS signals. It is this variation in theinput signal levels that the AGC technique of the present inventionaccounts for without requiring the use of prohibitively expensive A/Dconverters.

[0025]FIG. 2 is an exemplary block diagram of the IF subsystem 18 moduleillustrated in FIG. 1. As shown in FIG. 2, the IF subsystem includes avariable gain amplifier (VGA) 32, a programmable gain amplifier (PGA)34, a quadrature downconversion circuit 36, filters 38, 39 and AIDconverters 40, 41. The incoming data signal is first coupled to the tothe variable gain amplifier 32 from the ODU via the IFL. As explained inmore detail below, the variable gain amplifier 32 functions tocompensate for long term power level variations in the received signal.In accordance with the present embodiment, the VGA 32 is initiallyadjusted based on the power level of the received Beacon signal suchthat the power level at the input of the A/D converters 40, 41 is at thedesired value. As noted above, and explained below, the control signalfor controlling the VGA 32 is generated by the modem subsystem 20. Theoutput of the VGA 32 is coupled to the input of the PGA 34. As explainedbelow, in the current embodiment, the PGA 34 is controlled in order tocompensate for the dynamic burst-to-burst changes which result fromreceipt of CONUS and PTP signals. In other words, when it is known thata PTP signal (or CONUS signal) is being received by the satelliteterminal 10, the PGA 34 is adjusted so as to negate the additionalsignal power contained in the PTP signal relative to the desired signallevel, which is based on the beacon power level. The output of PGA 34 iscoupled to the quadrature downconversion circuit 36, which functions todownconvert the IF signal into baseband I and Q data channels. Both theI and Q data channels are coupled to a respective filter 38, 39 and A/Dconverter 40, 41.

[0026] Accordingly, by utilizing the IF subsystem 18 illustrated in FIG.2, it is possible to maintain the power level input into the AIDconverters 40, 41 within a predefined range, and more importantly, allowfor the processing of input signals having a wide dynamic range withouthaving to utilize expensive, high performance A/D converters. Tosummarize, the VGA 32 is utilized to compensate for long term powerlevel variations (e.g., system performance variations), while the PGA 34is utilized to dynamically compensate for changes in input power levelson a burst-to-burst basis.

[0027]FIG. 3 is an exemplary block diagram of the VGA controller 50contained in the modem subsystem 20, which is utilized to generate theVGA control signal 33 depicted in FIG. 2. It is noted that in thecurrent embodiment, the VGA control signal 33 is generated on acontinuous basis so as to allow the VGA 32 to adjust for variations inthe power level of the beacon signal. The VGA controller 50 has twomodes of operation, namely, an acquisition mode and a tracking mode.

[0028] In the acquisition mode (i.e., when the satellite terminal isfirst attempting to receive incoming data signals), the gain of the VGA32 is initially set to its maximum value, and the gain of the PGA 34 isset to a predetermined nominal value, which is expected to allow receiptof the beacon signal. Once the VGA 32 and PGA 34 are initialized,sampled data from the IF subsystem 18 is coupled to a power measurementcircuit 51, which functions to measure power for a given frame and tointegrate the received power for each half-slot for the given frame. Thepower measurement circuit 51 also receives a half-slot indicator signal52 as an input signal. The output of the power measurement circuit 51 iscoupled to a peak detector circuit 53, which functions to store the peakpower level output by the power measurement circuit 51. The output ofthe peak detector circuit 53 serves as a first input to an errordetector 54. The second input to the error detector 54 is apredetermined power reference signal corresponding to theexpected/desired power level. The predetermined power level can be basedon, for example, the nominal beacon signal value. The error detector 54functions to generate an error signal representing the differencebetween the peak power level of the incoming signal and the powerreference signal. The error signal output by the error detector 54 isthen coupled to a loop-filter 56 via a multiplexer 55. The loop-filter56, which in the current embodiment, is a standard first-order filter,functions to smooth frame-frame variations in the power measurements.The bandwidth of the loop filter 56 is preferably chosen so as to allowtracking of the maximum rate of change of the input signal level, whileat the same time providing sufficient filtering so that the power leveldoes not fluctuate too greatly on a frame-to-frame basis. The output ofthe loop filter 56 is then coupled to a modulator 57 (e.g., sigma-deltamodulator), which functions to modulate a carrier signal with the errorsignal. The output of the modulator 57 functions as the VGA controlsignal 33 illustrated in FIG. 2.

[0029] It is noted that in the given embodiment, in the acquisitionmode, the VGA controller 50 initially functions to determine the highestinput power level and then sets the VGA 32 based on this power readingso that the incoming PTP signals (which have the highest power rating)do not overload the A/D converters 40, 41.

[0030] Once the initial acquisition has been completed, the VGAcontroller 50 enters a tracking mode of operation (and as explainedbelow the PGA controller begins to dynamically control the PGA so as toadjust the amplification based on the data type contained in eachframe). In the tracking mode, the operation of the VGA controller 50 issimilar to the operation in the acquisition mode, with the distinctionthat in the tracking mode, the error signal fed to the loop-filter 56 isgenerated based on the difference (i.e., correlation) between a Beaconunique word power measurement and a unique word reference signal, whichcorresponds to the desired/expected value of the beacon signal. Thus,the error detector 59 comprises a UW correlator. External beaconcorrelation circuitry supplies the unique word “UW” power value to theerror detector input 59. Once again, the object of the VGA controller 50in the tracking mode is to adjust the VGA 32 gain so as to maintain thebeacon UW power at a predetermined value, which in the given embodiment,is between −6 and −15 dBFS. It is noted that the use of unique wordcorrelations to determine differences in power levels is well known inthe art, and therefore not described in further detail herein. However,it is also noted that the power measurements performed by the presentinvention are in no way limited to unique word correlation processes.Any suitable means of measuring and comparing the power levels can beutilized.

[0031] It is noted that in the event that the A/D converter 40, 41 isoverdriven, the output of the UW correlator may not reflect the truevalue of the beacon power. If the A/D overflow indicator 58 is activeduring the beacon UW time interval, a negative offset is added to theerror detector output value to force the VGA 32 gain lower. The offsetis cleared at the end of the given frame.

[0032]FIG. 4 is an exemplary block diagram of the PGA controller 62contained in the modem subsystem 20, which is utilized to generate thePGA control signal 23 depicted in FIG. 2. As stated above, the PGAcontroller 62 functions to dynamically adjust the amplification of thePGA 34 in the IF subsystem 18 to compensate for changes in the signallevels associated with the C3, C4 and PTP signal modes relative to thebeacon signal level.

[0033] Referring to FIG. 4, in the embodiment shown therein, the PGAcontroller 62 comprises a separate power detector 63, 64, and 65 fordetermining the power level of the incoming PTP, C3 and C4 data signals,respectively. Similar to the error detector 59 utilized in the AGCcontroller 50, the power detectors 63, 64 and 65 of the PGA controller62 utilize UW correlation techniques to determine the power level of therespective incoming signal. The PGA controller 62 further includes gaincomputation circuits 66, 67, and 68 each of which, function to set theattenuation of the PGA 34 based on the type/mode of the received signal.As explained below, the gain computation circuits can also be utilizedfor adaptive adjustment of the amount of attenuation utilized so as tocompensate for variations in the received signal levels due to, forexample, rain fade, path loss, etc. The PGA controller 62 furthercomprises a multiplexer 69 which receives the outputs of the gaincomputation circuits and functions to select which of the outputs iscoupled to the PGA 34. The multiplexer 69 further comprises an input forthe beacon gain, which is utilized as a reference signal to set the PGA34 when the other data modes are not being received by the satelliteterminal 10.

[0034] In operation, the satellite terminal 10 knows a priori thesignaling modes contained in a given incoming data frame. As such, priorto processing a given slot having a known signal mode, the PGAcontroller 62 functions to control the PGA to select the predeterminedamount of attenuation associated with the given data mode. For example,assuming the slot contained a PTP signal, the PGA controller 62 wouldfunction to control the attenuation of the PGA 34 such that the powerlevel of the PTP signal was reduced to substantially the desired nominalpower level (which in the current embodiment is based on the nominalvalue of the beacon signal). In other words, upon receipt of a givendata mode, the PGA 34 is controlled so as to have an attenuation valueequal to the value the power level of the incoming signal is expected tobe above the nominal value, thereby negating the increase in power levelassociated with the given data mode. As a result, the power level of thesignal input into the A/D converters 40, 41 can be maintained atsubstantially the same level regardless of the type of received signal.It is noted that the required amount of attenuation necessary for agiven type of signal can be stored in memory and recalled as necessaryduring operation. For example, the predetermined amount could simply bestored in the respective gain/attenuation circuit 66, 67 and 68 andoutput as the PGA control signal 23 as appropriate.

[0035] Returning to FIG. 4, as mentioned above, the embodiment of thePGA controller 62 illustrated in FIG. 4 also allows for the adaptiveadjustment of the PGA attenuation values so as to compensate/adjust forvariations in the power levels of the different signal modes (e.g., PTP,C3, C4) over time. The operator will be able to select betweenautomatically adjusted values determined from the algorithm below, orthe adaptation algorithm may be disabled and the attenuator will utilizesoftware-programmable fixed values loaded into a register as describedabove.

[0036] One embodiment of an adaptive adjustment routine is as:

[0037] 1. Nominal PGA gain values for BCN, C4, C3, and PTP bursts aredetermined by link budgets a priori, and are used as initial PGAcontroller values.

[0038] 2. The UW power measurement is monitored for each type of burst,i.e., C4, C3, PTP, for which a valid UW detection occurs. Referring toFIG. 4, this step is accomplished by the respective power detectors 63,64, and 65, each of which receive a UW power measurement and a microcell72 (PTP only) and UW valid indicator signal 73 as input signals. Asnoted below, if the microcell (PTP only) and UW valid indicators do notconfirm that the signal being processed was intended for the givensatellite terminal, no adaptation or adjustment of the attenuation valueoccurs.

[0039] 3. For each type of burst, if the UW power measurement is greaterthan the greatest UW power measured to that point for the given frame,the new maximum power measurement is stored in the appropriate maximumpower register.

[0040] 4. For each type of burst, if the UW power measurement is lessthan the smallest UW power measured to that point for the given frame,the new minimum power measurement is stored in the appropriate minimumpower register.

[0041] 5. For each type of burst, if the A/D out-of-range signal 74becomes active during any burst in the frame, then the appropriateout-of-range indicator is set.

[0042] 6. At the end of the frame, the maximum and minimum power valuesare compared to fixed predetermined limits, for each burst type (i.e.,PTP, C3 and C4).

[0043] 7. If the out-of-range indicator is set, the PGA gain value forthat type of burst is adjusted downward by one step (2 dB) and stored inthe appropriate register.

[0044] 8. If the maximum power register value is greater than themaximum power threshold, the PGA gain value for that type of burst isadjusted downward by one step (2 dB) and stored in the appropriateregister.

[0045] 9. If the minimum power register value is less than the minimumpower threshold, the PGA gain value for that type of burst is adjustedupward by one step (2 dB) and stored in the appropriate register.

[0046]10. At the start of the next frame, the new gain values becomevalid, i.e. the pending gain values are transferred to the current gainregisters, and the out-of-range indicators are cleared

[0047] Thus, the PGA controller 62 of the present invention also allowsfor compensation of the predetermined attenuation values associated withthe different data modes. It is noted that each of the gain computationcircuits also receive an input signal 79 indicating the start and stoppoints of the frames.

[0048] As described above, the system and method of providing automaticgain control in accordance with the present invention provides importantadvantages over prior art devices. Most importantly, the AGC system andmethod of the present invention allows for the compensation/processingof a wide dynamic range regarding the input signal without requiring theuse of a fast, high-order bit A/D converter in the demodulator. Asnoted, the use of such an A/D converter (assuming it is available) wouldrender the overall system prohibitively expensive due to the cost ofsuch an A/D converter. In contrast, as the present invention allows forthe compensation of a large dynamic range utilizing a low-cost, readilyavailable converter, the present invention results in a commerciallyviable system.

[0049] Of course, it should be understood that a wide range of otherchanges and modifications can be made to the preferred embodimentdescribed above. It is therefore intended that the foregoing detaileddescription be regarded as illustrative rather than limiting and that itbe understood that it is the following claims including all equivalents,which are intended to define the scope of the invention.

What is claimed is:
 1. An apparatus for providing automatic gain controlfor use in a satellite terminal of a satellite communication system,said satellite communication system capable of transmitting a pluralityof different modes of data, said apparatus comprising: a demodulatorcircuit having an analog to digital converter; a first variableattenuator having an attenuation value set on the basis of a measuredpower level of a predetermined data signal; and a second variableattenuator having an attenuation value set on the basis of the mode ofdata being received by said satellite terminal, each of said data modeshaving a corresponding predetermined attenuation value associatedtherewith which is utilized as the attenuation value of said secondvariable attenuator when said satellite terminal receives said datamode.
 2. The apparatus of claim 1, wherein said first variableattenuator and said second variable attenuator are operative formaintaining the input power level to said analog to digital converterwithin a predetermined range.
 3. The apparatus of claim 1, wherein saidfirst variable attenuator comprises a variable gain amplifier, saidvariable gain amplifier having an input control signal representing themeasured power level of said predetermined data signal, said inputcontrol signal being updated such that said variable gain amplifiercompensates for variations in the power level of said predetermined datasignal over time.
 4. The apparatus of claim 3, wherein said inputcontrol signal comprises a modulated signal which is continuously fed tosaid variable gain amplifier, said modulated signal being generated by acircuit operative for tracking changes in the power level of saidpredetermined data signal during operation of said satellite terminal.5. The apparatus of claim 1, wherein said second variable attenuatorcomprises a programmable gain amplifier, said programmable gainamplifier be programmed to the predetermined attenuation valuecorresponding to the data mode of the data being processed by saiddemodulator.
 6. The apparatus of claim 5, wherein said data mode of thedata to be received by the demodulator is known a priori such that saidprogrammable gain amplifier can be programmed to the predeterminedattenuation value corresponding to the given data mode prior to saiddemodulator processing such data.
 7. A method for providing automaticgain control for use in a satellite terminal of a satellitecommunication system, said satellite communication system capable oftransmitting a plurality of different modes of data, said methodcomprising the steps of: measuring a power level of a predetermined datasignal received by said satellite terminal, adjusting an attenuationvalue of a first variable attenuator on the basis of said measured powerlevel of said predetermined data signal, adjusting an attenuation valueof a second variable attenuator on the basis of the mode of data beingreceived by said satellite terminal, each of said data modes having acorresponding predetermined attenuation value associated therewith whichis utilized as the attenuation value of said second variable attenuatorwhen said satellite terminal receives said data mode, wherein said firstvariable attenuator and said second variable attenuator are operativefor maintaining the input power level to an analog to digital convertercontained in a demodulator of said satellite terminal within apredetermined range.
 8. The method of claim 7, wherein said firstvariable attenuator comprises a variable gain amplifier, said variablegain amplifier having an input control signal representing the measuredpower level of said predetermined data signal, said input control signalbeing updated such that said variable gain amplifier compensates forvariations in the power level of said predetermined data signal overtime.
 9. The method of claim 8, wherein said input control signalcomprises a modulated signal which is continuously fed to said variablegain amplifier, said modulated signal being generated by a circuitoperative for tracking changes in the power level of said predetermineddata signal during operation of said satellite terminal.
 10. The methodof claim 7, wherein said second variable attenuator comprises aprogrammable gain amplifier, said programmable gain amplifier beprogrammed to the predetermined attenuation value corresponding to thedata mode of the data being processed by said demodulator.
 11. Themethod of claim 10, wherein said data mode of the data to be received bythe demodulator is known a priori such that said programmable gainamplifier can be programmed to the predetermined attenuation valuecorresponding to the given data mode prior to said demodulatorprocessing such data.
 12. An apparatus for providing automatic gaincontrol for use in a satellite terminal of a satellite communicationsystem, said satellite communication system capable of transmitting aplurality of different modes of data, said apparatus comprising: meansfor measuring a power level of a predetermined data signal received bysaid satellite terminal, means for adjusting an attenuation value of afirst variable attenuator on the basis of said measured power level ofsaid predetermined data signal, means for adjusting an attenuation valueof a second variable attenuator on the basis of the mode of data beingreceived by said satellite terminal, each of said data modes having acorresponding predetermined attenuation value associated therewith whichis utilized as the attenuation value of said second variable attenuatorwhen said satellite terminal receives said data mode, wherein said firstvariable attenuator and said second variable attenuator are operativefor maintaining the input power level to an analog to digital convertercontained in a demodulator of said satellite terminal within apredetermined range.
 13. The apparatus of claim 12, wherein said firstvariable attenuator comprises a variable gain amplifier, said variablegain amplifier having an input control signal representing the measuredpower level of said predetermined data signal, said input control signalbeing updated such that said variable gain amplifier compensates forvariations in the power level of said predetermined data signal overtime.
 14. The apparatus of claim 13, wherein said input control signalcomprises a modulated signal which is continuously fed to said variablegain amplifier, said modulated signal being generated by a circuitoperative for tracking changes in the power level of said predetermineddata signal during operation of said satellite terminal.
 15. Theapparatus of claim 12, wherein said second variable attenuator comprisesa programmable gain amplifier, said programmable gain amplifier beprogrammed to the predetermined attenuation value corresponding to thedata mode of the data being processed by said demodulator.
 16. Theapparatus of claim 15, wherein said data mode of the data to be receivedby the demodulator is known a priori such that said programmable gainamplifier can be programmed to the predetermined attenuation valuecorresponding to the given data mode prior to said demodulatorprocessing such data.